The invention relates to a method for determining at least one optical property of an imaging optical system which is designed to image an object disposed in an object plane of the optical system into an assigned image plane. Moreover, the invention relates to an apparatus for determining at least one optical property of an imaging optical system which is designed to image an object disposed in an object plane of the optical system into an assigned image plane. Moreover, the invention relates to a microlithography exposure tool with an illumination system and/or a projection objective and an apparatus of this type.
An optical property of an imaging optical system which can be determined by means of this type of method or this type of apparatus can for example be the numerical aperture or the telecentricity of the optical system. The numerical aperture of an optical system is a dimension-free number which describes an angular region for the inlet and for the outlet of electromagnetic radiation into the optical system. The numerical aperture of an optical system, such as for example of a projection objective of a microlithography exposure tool is defined as follows:NA=n×sin θ  (1)n being the refractive index of the medium surrounding the optical system and θ being half the opening angle of the maximum inlet cone for electromagnetic radiation passing into the optical system or of the maximum outlet cone for radiation leaving the optical system.
Previously known methods for determining the numerical aperture of an imaging optical system measure the electromagnetic radiation passing out of the system by means of an interferometer. The precision of the determination of the numerical aperture of the optical system by means of the interferometric method is limited, however. Measuring the variation in the numerical aperture across the image field of the optical system is also very laborious here.
It is known that determining the telecentricity of an optical system serves to identify deviations of the telecentricity characteristics of optical imaging systems from their ideal telecentricity characteristics of optical imaging systems, i.e. telecentricity errors. With an imaging optical system which is associated with a telecentricity error the main beam for a respective field point does not extend, as in the error-free case, parallel to the optical axis of the imaging system, but rather tilted in relation to the latter, the tilt angle being a quantitative measure of the telecentricity error. It is obvious to determine the telecentricity error here by the energetic centre point position of the image of a respective field point in the image plane of the optical system being measured in different focus adjustments and from this the tilt angle is calculated trigonometrically. Opposing this, however, is the difficulty that the energetic centre point position of the image of a respective field point can also vary in the image plane independently of the focus adjustment due to other image errors with which imaging systems are typically associated, such as for example coma and image field curvature errors. Consequently, from this type of measurement of the energetic centre point position in the image plane as a function of the focus setting alone one can only draw conclusions regarding the telecentricity error with a high level of imprecision.